Embedded in the wall of 111 Cannon Street, in the City of London, just above the sidewalk, is a small grilled window that appears like a decorative skylight for an underground basement. Behind it, in a glass enclosure and dimly lit from inside, is an irregular chunk of oolitic limestone, believed to be one of London’s most ancient and important relic. But nobody remembers what it was used for.

Ignored by the thousands of Londoners who walk past the grilled window every day, the London Stone has stood in or around the same spot on present-day Cannon Street for at least a thousand years, and possibly even two. The stone’s mysterious origin has fascinated people for centuries and even appeared in the works of Shakespeare, William Blake and Dickens.